PROBLEM SOLVING TACTICS FORCE AND MOTION
Tactic 6: Normal Force
For the normal force on a body holds only when is directed upward and the body’s vertical acceleration is zero, so we do apply it for other orientations of N or when the vertical acceleration is not zero. Instead, we must derive a new expression for from Newton’s second law.
We are free to move N around in a figure as long as we main its orientation. For example, in we can slide it downward so that its head is at the boundary of the body and the tabletop. However, N is least likely to be misinterpreted if its tail is at that boundary or somewhere within the body (as shown). An even better technique is to draw a free body diagram as in, with the tail of N directly on the dot or sketch representing the body.